The flow capacity of the subject control valves must be selected so as to be suitable for each control object. Also, the flow characteristics of the valves must be compatible with the heat transfer characteristics of the control object. Basic ball valves or plug valves have a certain inherent flow characteristic not ideally suited for the above mentioned applications. Also, the flow capacity of such valves, as compared to the size of the valve, is very high. The use of reducing couplings and installing a valve with a smaller size than the basic pipe size tends to further distort the valve characteristics.
U.S. Pat. No. 4,193,580 describes a plug valve which has a special body shape. Among other things, there is brief mention in the patent of ports and seats with a specially shaped opening to give the valve desired flow characteristics. One advantage with the valve disclosed in the '580 patent is that the special opening or seat can be exchanged with another seat which has a differently shaped opening and therefore, gives the valve a different flow capacity and/or flow characteristics. This makes it possible to adapt the disclosed valve for a specific control object. However, in order to change the seats, the valve body has to be taken apart. Therefore, valves that are bolted together and easy to open are preferred, or even necessary, for practice of the disclosure. So called "three piece" ball valves are especially well suited. The three pieces are bolted together, providing easy access to the seats.
A significant disadvantage of the plug valve taught in the '580 patent is that the HVAC market is in general very price sensitive, and therefore relatively inexpensive "two piece" ball valves are often used. The two pieces of such valves are screwed together, and the threads are secured by an epoxy or similar substance. This makes it very difficult (indeed, practically impossible) to take the valve apart in order to exchange the seats. Specifically, if a "two piece" valve is taken apart and reassembled in the field, there is a risk that it may be tightened too little or too much. This will either cause leakage through the seats, or an excessive torque to operate the valve.
An important consideration is that the above mentioned specially shaped seat has to perform two functions. It must provide a tight shut-off when the valve is closed and also provide the desired flow characteristics, when the valve is operated between opened and closed. What is desired, therefore, is a control valve which is inexpensive to manufacture, which is quick and easy to install and change, and which has flow capacity and flow characteristics which can be easily changed without disturbing the integrity of the valve seal.